1. Field of the Invention
The present invention relates to an ND (neutral density) filter used for a light amount adjusting device suitable for an imaging apparatus such as a video camera or a digital still camera and a method of manufacturing the ND filter, and to a technique capable of suppressing deterioration of optical performance even in an image pickup element having a small pixel pitch.
2. Related Background Art
A light amount adjusting device (diaphragm device) that changes a diameter of a diaphragm aperture formed by a plurality of diaphragm blades to adjust a light amount is used for an imaging optical system of an optical device such as a video camera. In such a diaphragm device, in the case where the aperture diameter becomes too small at the time when imaging is conducted on a high intensity object, deterioration of optical performance due to light diffraction becomes a problem.
Therefore, in order to prevent the aperture diameter from becoming too small even in the case where the imaging object is light, a light amount adjusting device using diaphragm blades in combination with an ND filter has been proposed and used in practice.
A diaphragm device disclosed in Japanese Patent Application Laid-Open No. H05-281593 is as follows. An ND filter is bonded to diaphragm blades to be positioned r in a diaphragm aperture formed by the diaphragm blades. The ND filter has a first region to which uniform transmittance is set and a second region whose transmittance successively changes. When the diaphragm aperture of the diaphragm device becomes a set small diaphragm aperture, only the first region of the ND filter is positioned in the diaphragm aperture formed by the diaphragm blades.
A diaphragm device disclosed in Japanese Patent Application Laid-Open No. S52-117127 is as follows. A fully-opened state of a diaphragm aperture to a state thereof with a predetermined aperture area is achieved by moving mechanical diaphragm blades. In small diaphragm control of no more than a predetermined diaphragm value, an ND filter whose light transmittance successively changes according to a density is approached to an aperture such that a filter portion having high transmittance first reaches the aperture.
According to Japanese Patent Application Laid-Open No. 2000-106649, the influence of a diffraction phenomenon on optical performance, resulting from the transmittances of an ND filter having a plurality of density regions, is described and an image pickup apparatus having an exposure control mechanism with measures against the influence being taken is disclosed.
In the cases of these conventional proposals, with respect to a main factor for the deterioration of optical performance in an intermediate diaphragm state of a diaphragm aperture, which includes a fully-opened state to a small diaphragm state, it is considered that the influence of diffraction resulting from a difference of the transmittances of the ND filter covering the aperture portion formed by the diaphragm blades is dominant. Thus, in the case of the ND filter having the plurality of density regions, measures against the influence of diffraction with attention to transmittances of the respective density regions and aperture areas have been proposed.
However, a factor for the deterioration of optical performance in the intermediate diaphragm state includes not only the influence of diffraction resulting from the difference of the transmittances of the ND filter but also the large influence of a phase difference of transmission wave front resulting from a thickness of the ND filter.
In this point, a phenomenon in which optical performance deteriorates in the case where a part of a diaphragm aperture portion is covered with a thick filter has been experimentally known. However, an example in which the influence of the thickness of the filter on the optical performance is analyzed and the concrete measures are taken has not been known.
According to Japanese Patent Application Laid-Open No. H06-265971, a structure has been proposed in which an ND filter having a transparent portion and a portion whose transmittance successively or stepwise changes is moved with a state in which the ND filter covers the entire fixed circular diaphragm aperture, thereby adjusting a light transmission amount, as a measure for avoiding the influence of the thickness of the ND filter on the optical performance.
The invention described in Japanese Patent Application Laid-Open No. H06-265971 focuses attention on only a large phase difference produced between a portion in which light transmits through an aperture and a filter member and a portion in which light transmits through the aperture but does not transmit through the filter member (transmits through a plain portion). It is described in Japanese Patent Application Laid-Open No. H06-265971 that the large phase difference becomes an aberration, thereby deteriorating imaging performance. However, there is no description with respect to a phase difference of transmission wave front of light transmitted through the filter member and whose density changes. In other words, there are no suggestions with respect to problems that arise in the case of actually realizing an ND filter in which transmittances change, and measures therefor, the problems being related to a minute phase difference of transmission wave front (equal to or less than a light wavelength), resulting from a minute change in thickness or a minute change in refractive index of the ND filter which will be caused in order to provide a change in transmittance.
According to the study made by the inventors of the present invention, it was found that a minute phase difference of transmission wave front equal to or less than the order of light wavelength greatly influences the optical performance under a certain condition.
Also, it was found that the appearance of the influence of the phase difference of transmission wave front on the optical performance is different from that of the influence of a density difference between adjacent transmittance regions of the ND filter on the optical performance.
Accordingly, the optical performance is greatly influenced under a certain condition due to the synergism of two components, the phase difference of transmission wave front and the density difference.